Organic peroxides are important for use as free radical producing initiators in the polymerization field, and particularly in the polymerization of ethylenically unsaturated monomers, such as vinyl chloride. There are numerous classes of organic peroxides. Commercially important classes of organic peroxides are dialkyl peroxides, diacyl peroxides, peroxydicarbonates, and peroxyesters.
A frequently employed industrial method for the synthesis of dialkyl peroxides is the alkylation of hydroperoxides with alcohols, olefins, esters, halides or epoxides (Ullmann's Encyclopedia of Industrial Chemistry, 4th ed., VCH, 1991, Vol. 19, pp. 205; J. Sanchez' T. N. Myers, in Kirk-Othmer Encyclopedia of Industrial Technology, 4th ed., Wiley, 1996, pp. 248–252). Reaction conditions depend on the nature of the reactants and usually involve acid or base catalysis. A typical industrial method for the synthesis of diacyl peroxides is the reaction of acyl halides or carboxylic acid anhydrides with hydrogen peroxide or an alkali metal peroxide (Ullmann's Encyclopedia of Industrial Chemistry, 4th ed, 1991, Vol 19, pp. 211–212; J. Sanchez; T. N. Myers, Kirk-Othmer Encyclopedia of Industrial Technology, 4th ed., Wiley, 1996, pp. 280–283).
An important industrial method for the synthesis of organic peroxyesters is the reaction of carboxylic acid halides, particularly chloride, with hydroperoxides. (In Ullmann's Encyclopedia of Industrial Chemistry, 4th ed. VCH, 1991, Vol. 19, pp. 216.) The process is usually carried out with high selectivity under Schotten-Baumann conditions using either organic or inorganic bases in aqueous or aqueous-organic media. Batch processing is generally employed when relatively small production volumes are required, whereas semi-continuous and continuous processing are employed when larger production volumes are required and when safety is a primary issue. (J. Sanchez; T. N. Myers, in Kirk-Othmer Encyclopedia of Industrial Technology, 4th ed. Wiley, 1996, Vol. 18, pp. 292–293; P. M. Kohn, Chem. Eng. 1978, Jul. 17, 1988–1989; U.S. Pat. No. 4,075,236.) In the case of preparing the peroxyesters in aqueous-organic media using aqueous alkali, phase transfer catalysis was developed to speed up the reaction at lower temperature. (S. Baj; A Chrobok; Polish J. Chem. 1999, 73, 1185–1189.)
Organic peroxides are typically made in large batches and sold in pure form, either as neat or diluted products. Polymer producers must store large quantities of organic peroxides for use in their polymerization processes. Precautions must be taken with the storage and handling of these materials as they are unstable and are sensitive to both thermal and impact shock and can detonate under certain conditions. Complying with all of the safety requirements of handling these materials results in the organic peroxides being very expensive to employ in the manufacture of polymers.
Various solutions to this problem have been proposed in the past. U.S. Pat. No. 4,359,427 proposes a process to continuously produce and purify peroxydicarbonates on the polymerization site and to store them in a diluted phase until used. Another approach that has been suggested is to produce organic peroxides in a large polymerization vessel before adding the polymerizable monomer. This is sometimes referred to as in-situ synthesis. Making the organic peroxide in a large vessel has resulted in quality problems for the polymer being produced for several reasons. One such reason is that there is not adequate mixing of the small amount of reactants in a large reactor vessel. Without adequate mixing, the reaction to form the organic peroxide is inefficient and the yield of organic peroxide produced varies, thus making the polymerization reactions using the organic peroxide initiators vary in reaction times. To make greater volumes, diluents are often used, such as solvents and water. With these diluents there is poor conversion of the reactants resulting in large amounts of undesirable by-products which are formed and which remain in the large reactor to contaminate the polymer that is ultimately produced in the reactor. Solvent dilution results in solvent being present which must be recovered and contaminates the recovery system for recovering unreacted monomer. Also, by making the organic peroxides in large polymerization vessels, productivity is lowered because the polymerization vessel is occupied with the organic peroxide synthesis process before each batch of polymer can be produced.
Great Britain Patent 1,484,675 proposes to solve these problems by producing peroxydicarbonates outside of the polymerization vessel in the presence of a solvent to obtain adequate mixing of the reactants. This method is undesirable because the solvent must be removed or else it becomes a contaminant in the polymerization process and contaminates the polymerization process monomer recovery system.
WO 97/27229 patent application proposes to solve the problem by making peroxydicarbonates outside of the polymerization reactor in a two-step process and adding a water insoluble liquid dialkyl alkanedicarboxylate. The dialkyl alkane dicarboxylate is a plasticizer for the resulting polymer and is undesirable in rigid applications of the polymer. Also, the two-step process is cumbersome and requires excess equipment.
U.S. Pat. No. 4,359,427, Great Britain patent 1,484,675 and WO 97/27229 all teach that peroxydicarbonates can be produced by reacting a chloroformate with an alkali metal peroxide.
U.S. Pat. No. 5,962,746 discloses production of organic peroxides by reacting a hydroxide, a peroxide, and an acyl halide under continuous vigorous agitation conditions. For example, disclosed is the batch-wise synthesis of diisobutyryl peroxide from isobutyryl chloride and either (1) ammonium hydroxide and hydrogen peroxide or (2) potassium hydroxide and hydrogen peroxide, using high power ultrasonication over a short reaction time in a hexane-water medium. These methods produced diisobutyryl peroxide in relatively low yields of 47% and 29% respectively.
The synthesis of diisobutyryl peroxide on a batch-wise basis from isobutyryl chloride and sodium peroxide using a magnetically stirred, two-phase reaction mixture at 25–30° C. was reported in M. Ravey, J. Poly. Sci. Poly. Chem. Ed., 15 1977, pp. 2559–2570. This disclosure reports that low yields of product (approximately 50%) only were achieved, and the reduced yield was attributed to hydrolysis of the isobutyryl chloride under the reaction conditions.
The in-situ synthesis of diisobutyryl peroxide on a batch-wise basis from isobutyric anhydride in mechanically tumbled reactors, with yields of around 67% was reported in J. A. Barter, D. E. Kellar, J. Poly. Sci. Poly. Chem. Ed., 15, 1977, pp. 2545–2557. Under more concentrated conditions, in a two-phase reaction system, isobutyryl chloride in hexane was slowly added to an aqueous solution of sodium peroxide, followed by a short period of rapid agitation, resulting in yields of diisobutyryl peroxide of around 72%. The use of isobutyryl chloride as a starting material under in-situ reaction conditions resulted in very low yields of the desired diisobutyryl peroxide.